1. Technical Field
This invention relates to measuring devices and, more particularly, to an electrometer for measuring DC voltage potentials.
2. Discussion
Direct current voltage sources are used in a wide variety of different applications. In one application, electric vehicles include a battery pack with a plurality of batteries connected in series to power the electric vehicle. To ensure proper charging and maintenance of the batteries, each battery voltage is individually and accurately measured. Because the batteries are connected in series each battery floats at a different voltage relative to ground. Some batteries float at hundreds or even thousands of volts above ground potential. This known as the common mode potential (V.sub.COMMON-MODE).
It is often difficult to obtain an accurate measurement of each individual battery's potential in the battery pack under circumstances where the battery to be measured is at a high common mode potential. Quite often it is necessary to utilize a sensing wire having a very large series resistance in such cases. Since the series resistance required is proportional to the square of the voltage, the resistance value of the series resistance increases very quickly with increasing V.sub.COMMON-MODE voltages. For example, if the current is to be limited to 4 milliamps when the V.sub.COMMON-MODE voltage is 250 volts, the series resistance must be at least 62.5 kohms.
In the past, electrometers for making such measurements have typically used electromechanical meter-type displays for displaying the measured voltage. it was also impractical to use such electrometers with microprocessors or other semiconductor devices without the use of isolation transformers to isolate the high V.sub.COMMON-MODE voltage from the semiconductor devices. The isolation transformers add cost, increase size and are vibrationally sensitive.
Therefore, a simple, durable, relatively inexpensive, accurate and compact electrometer which can be interfaced with semiconductor devices would be desirable.